Nitride compound semiconductor light emitting device and method for producing the same

ABSTRACT

A nitride compound semiconductor light emitting device includes: a GaN substrate having a crystal orientation which is tilted away from a &lt;0001&gt; direction by an angle which is equal to or greater than about 0.05° and which is equal to or less than about 2°, and a semiconductor multilayer structure formed on the GaN substrate, wherein the semiconductor multilayer structure includes: an acceptor doping layer containing a nitride compound semiconductor; and an active layer including a light emitting region.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a nitride compound semiconductorlight emitting device formed on a GaN substrate, and a method forproducing the same.

[0003] 2. Description of the Related Art

[0004] Conventionally, nitride compound semiconductor light emittingdevices have been studied or utilized as light emitting devices or highpower devices. In the case of light emitting devices, for example, lightemitting devices covering a wide range of colors from blue to orange canbe technically realized by using nitride compound semiconductors ofvarious compositions. By taking advantage of such properties of nitridecompound semiconductors, blue and green light emitting diodes (LEDs)have been realized in recent years. As for laser devices, blue-violetsemiconductor laser devices are under development.

[0005] By using a nitride compound semiconductor, a light emittingdevice is typically produced as follows. A current injection layerhaving n-type properties is formed on a substrate, e.g., amirror-polished sapphire (0001) substrate, upon which a nitride compoundsemiconductor is to be epitaxially grown. An active layer and a currentinjection layer containing an acceptor impurity are further formedthereupon. It is known that the use of a quantum well layer having athickness of about 10 nm or less for an active layer results in a highemission intensity. The emission wavelength can be varied by adjustingthe In (indium) component ratio in an InGaN active layer, for example.After the entire light emitting device structure has been formed, thedevice is subjected to a heat treatment in an N₂ gas, whereby theacceptor is activated, so as to impart p-type properties thereto. Thus,an LED or a laser device is completed.

[0006] In general, by doping an n-type nitride compound semiconductorcrystal with Si using a SiH₄ gas during a crystal growth process, forexample, an electron density of 10¹⁸ cm⁻³ or more can be easilyobtained. On the other hand, in order to obtain a hole density on theorder of 10¹⁸ cm⁻³ with a p-type nitride compound semiconductor crystal,it is necessary to dope the p-type nitride compound semiconductorcrystal with Mg using CP₂Mg (Bis(cyclopentadienyl)Magnesium) or EtCP₂Mg(Bis(Ethylcyclopentadienyl) Magnesium) during a crystal growth process,and after the entire light emitting device structure including an activelayer has been formed, subject the device to a heat treatment in aninert gas such as N₂. A p-crystal having a high hole density “as grown”has not hitherto been obtained.

[0007] As used herein, a “high hole density” means a density of 10¹⁷cm⁻³ or more. The expression “as grown” is used to describe a devicewhich, after crystal growth has taken place, has not been subjected to aheat treatment, electron beam irradiation, etc. An “acceptor dopinglayer” means a layer which has been doped with an acceptor impurity,e.g., Mg.

[0008] The reason why an acceptor doping layer does not exhibit a p-typeconductivity “as grown” under the conventional methods is that Mg atomswhich are taken into the mother crystal have been inactivated byhydrogen. Specifically, a nitride compound semiconductor crystal whichhas been formed on a conventional sapphire substrate has a highconcentration of defects and/or nitrogen vacancies due to a latticemismatch as high as 13% with the sapphire substrate. Therefore, Mg atomscannot be taken into the crystal by themselves, but rather are entrappedin an inactive state, i.e., Mg-H. Accordingly, in order to sever theMg-H bonds so as to obtain active Mg atoms, it is necessary to applythermal energy at a temperature on the order of several hundred ° C. inan inert gas atmosphere free of hydrogen after the light emitting devicestructure has been formed.

[0009] However, even after a heat treatment, which damages a thermallyunstable active layer containing In, the resultant hole density would bebetween a latter half of the 10¹⁷ cm⁻³ order to the 10¹⁸ cm⁻³ order. Toreduce the operation voltage in a light emitting device, it is necessaryto reduce a contact resistance when a p-type is electrode has beenformed. Therefore, there is a desire for achieving an increased holedensity in a p-type layer. In particular, a device which operates with ahigh current density, e.g., a laser device, is likely to be heated dueto a high contact resistance, so that degradation may begin from aninterface between the electrode and the p-type layer, leading toelectrode destruction. In addition, excessive heating might causedeterioration associated with the mobility of or increase indislocations within the light emitting device, resulting in a decreasein the emission intensity or fluctuation in the emission wavelength.Thus, the low hole density level in a p-type layer presently achievableunder the conventional technique is detrimental to the emissioncharacteristics and/or longevity of light emitting devices.

[0010] Moreover, a light emitting device formed on a conventionalsapphire substrate not only suffers from the inactivation of Mg, butalso receives unfavorable influences on an InGaN multiple quantum wellactive layer. As mentioned above, a light emitting device which includesan InGaN quantum well active layer formed on a sapphire substrate has asubstantially incommensurate lattice constant with that of the sapphiresubstrate, and hence has a high concentration of nitrogen vacanciesand/or threading dislocations, i.e., dislocations penetrating the devicefrom the substrate interface to the device surface through the quantumwell structure. In particular, a current which flows through a threadingdislocation is a component which does not contribute to emission, andtherefore increases the driving current density in the light emittingdevice, inducing heating within the light emitting device. Moreover,since a nitride compound semiconductor containing In is very unstable interms of chemical-thermal equilibrium during a crystal growth process, ahigh concentration of dislocations are present. In the presence of sucha high level of undulation in the underlying layer, each layer in themultiple quantum well structure formed thereon will have a non-uniformthickness.

[0011] As a method for solving the problems of dislocations and nitrogenvacancies, Japanese Laid-Open Patent Publication No. 9-23026 discloses atechnique of performing a two-part growth involving a buffer layer,where an angle between a sapphire substrate and the (0001) plane ismaintained equal to or less than 5°, thereby reducing dislocations andimproving emission characteristics. There is also disclosed a techniqueof, after growing a single quantum well active layer of InGaN,interrupting the growth or observing a watt period for 60 minutes orless to obtain a light emitting device having a uniform emission stateand high yield. Japanese Laid-Open Patent Publication No. 10-126006discloses that a quantum well laser device having a low thresholdcurrent density can be formed by forming a well layer to become anactive layer in a three-well quantum well structure, observing a waitperiod for 2 to 10 seconds, and then forming semiconductor layers.

[0012] However, in all of the aforementioned conventional techniques, aheat treatment for imparting p-type properties is required after forminga light emitting device structure. Due to an insufficient carrierdensity, a sufficiently low p-type contact resistance has not beenrealized. In addition, the problems associated with a heat treatment forimparting p-type properties, e.g., a damaged active layer, non-uniformcomposition of In-containing layers, non-uniform layer thicknesses,deterioration in crystal quality, etc., have not been solved. Therefore,it is difficult with conventional techniques to produce ahigh-efficiency LED or a low-threshold semiconductor laser device whichrequires a reduced operation voltage and/or current. Thus, there to aneed for a technique of producing light emitting devices having improvedcharacteristics.

SUMMARY OF THE INVENTION

[0013] A nitride compound semiconductor light emitting device accordingto the present invention includes: a GaN substrate having a crystalorientation which is tilted away from a <0001> direction by an anglewhich is equal to or greater than about 0.05° and which is equal to orless than about 2°, and a semiconductor multilayer structure formed onthe GaN substrate, wherein the semiconductor multilayer structureincludes: an acceptor doping layer containing a nitride compoundsemiconductor; and an active layer including a light emitting region.

[0014] In one embodiment of the invention, the acceptor doping layer iscomposed essentially of Ga_(x)In_(y)Al_(1−(x+y))N (where 0≦x≦1; 0 ≦y≦1;and 0≦x+y ≦1).

[0015] In another embodiment of the invention, the GaN substrate has acrystal orientation which is tilted away from a <0001> direction in a<11-20> or <1-100> direction.

[0016] In still another embodiment of the invention, the acceptor dopinglayer exhibits a p-type conductivity as grown.

[0017] In still another embodiment of the invention, the GaN substrateand the active layer are formed so as to be apart from each other by adistance which is equal to or greater than about 1 μm.

[0018] In still another embodiment of the invention, the active layerhas a quantum well structure, and the active layer has an averagedsurface roughness which is equal to or less than a thickness of a welllayer in the quantum well structure.

[0019] In still another embodiment of the invention, the active layerincludes at least one well layer and at least one barrier layer.

[0020] A method for, producing the nitride compound semiconductor lightemitting device according to the present invention includes: after atleast one of the at least one well layer and the at least one barrierlayer has been formed, observing a wait period during which no otherlayers are formed, the wait period having a predetermined length.

[0021] In one embodiment of the invention, the predetermined length ofthe wait period is equal to or greater than about 1 second and is equalto or less than about 60 minutes.

[0022] In another embodiment of the invention, the method furtherincludes: supplying a carrier gas into a chamber, in which the GaNsubstrate is placed, during the wait period after at least one of the atleast one well layer and the at least one barrier layer has been formed,the carrier gas containing nitrogen as a main component.

[0023] In still another embodiment of the invention, the method furtherincludes: supplying a carrier gas and a group V gas Into a chamber, Inwhich the GaN substrate is placed, during the wait period after at leastone of the at least one well layer and the at least one barrier layerhas been formed, the carrier gas containing nitrogen as a maincomponent.

[0024] Thus, the invention described herein makes possible theadvantages of providing a high-intensity nitride compound semiconductorlight emitting device which emits light with a low operation voltageand/or current.

[0025] These and other advantages of the present invention will becomeapparent to those skilled in the art upon reading and understanding thefollowing detailed description with reference to the accompanyingfigures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIG. 1A is a schematic cross-sectional view illustrating asemiconductor light emitting device produced by using a nitride compoundsemiconductor according to the present invention.

[0027]FIG. 1B is a schematic cross-sectional view illustrating thestructure of an active layer of a light emitting device producedaccording to the present invention.

[0028]FIG. 1C is a flow chart illustrating steps for producing an activelayer of a light emitting device according to one embodiment of theinvention.

[0029]FIG. 1D is a flow chart illustrating steps for producing an activelayer of a light emitting device according to another embodiment of theinvention.

[0030]FIG. 2 is a schematic cross-sectional view illustrating a mannerin which growth cores may form in a case of a substrate surface having arelatively large tilt angle.

[0031]FIG. 3 is a schematic cross-sectional view illustrating a mannerin which growth cores may form in a case where a tilt angle from the<0001> direction is defined according to the present invention, wherebysurface steps are optimized.

[0032]FIG. 4 is a schematic block diagram :illustrating a crystal growthapparatus which may be used in the present invention.

[0033]FIG. 5 is a graph illustrating a schedule of growth temperaturesin the vicinity of an active layer and flow rates of respectivematerials according to the present invention;

[0034]FIG. 6 is a graph illustrating a relationship between the tiltangle of a substrate surface and hole density according to the presentinvention.

[0035]FIG. 7 is a graph illustrating a relationship between the tiltangle of a substrate surface, threading dislocation density, andaveraged surface roughness according to the present invention.

[0036]FIG. 8 shows a relationship between the tilt angle of substratesurface and emission intensity of a semiconductor light emitting deviceaccording to the present invention.

[0037]FIG. 9 is a graph illustrating a relationship between the tiltangle of substrate surface and emission intensity of a semiconductorlight emitting device according to the present invention, where thegrowth temperature for an active layer is varied.

[0038]FIG. 10 is a graph illustrating a relationship between the totalthickness of underlying n-GaN layers and averaged surface roughness of auppermost growth surface according to the present invention.

[0039]FIG. 11 is a graph illustrating a relationship between the totalthickness of underlying layers and emission intensity of a semiconductorlight emitting device according to the present invention.

[0040]FIG. 12 is a graph illustrating a relationship between wait periodobserved after growing each barrier layer and emission intensity of asemiconductor light emitting device according to the present invention.

[0041]FIG. 13 is a graph illustrating a relationship between wait periodobserved after growing each well layer and emission intensity of asemiconductor light emitting device according to the present invention.

[0042]FIG. 14 is a graph illustrating a relationship between nitrogenpartial pressure, emission intensity, and emission wavelength of asemiconductor light emitting device according to the present invention.

[0043]FIG. 15 is a graph illustrating a relationship between the flowrate of NH₃ supplied during a wait period and emission intensity of asemiconductor light emitting device according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044]FIG. 2 presents a schematic microscopic observation of surfaces ofsemiconductor layers grown on a substrate 201 not satisfying a crystalorientation range constraint defined by the present invention. As seenfrom FIG. 2, there is a non-uniform distribution of steps 202 over thesubstrate 201, the steps 202 having a height on the order of a fewatomic layers. As a result, a three-dimensional growth mode (i.e. localprogress of growth) is predominant because non-uniform growth cores 203hinder an organized step flow growth. Thus, a high concentration ofthreading dislocations and/or nitrogen vacancies may occur with thegrowth of semiconductor layers in a thickness direction. Penetrationdislocations and/or nitrogen vacancies may result in entrapment of Mg-H.Moreover, a crystal having a high concentration of threadingdislocations and/or nitrogen vacancies has a substantially undulateduppermost surface. Such a surface does not provide an appropriateunderlying layer for forming a quantum well structure thereon. Inparticular, it may affect the crystallinity and composition uniformityof an light emitting layer containing In.

[0045] On the other hand, FIG. 3 presents a schematic microscopicobservation of surfaces of semiconductor layers grown on a substrate 301whose crystal orientation is tilted away from a <0001> direction by aslight angle in the range from about 0.05° to about 2° in a <11-20> or<1-100> direction. As shown in FIG. 3, steps 302 on the substrate 301are distributed in an optimum, uniform manner. As a result, atwo-dimensional growth mode occurs in which material species which havearrived at the substrate surface in a vapor phase repeat migration andrevaporization, forming a uniform distribution of growth cores 303 overthe entire substrate surface, whereby a layer-by-layer planar growthoccurs. Thus, the generation of threading dislocations and/or nitrogenvacancies is effectively reduced. Especially in the case of growing ap-type crystal, Mg atoms can be taken into is the crystal by themselves,so that a p-type crystal having a high hole density can be obtainedwithout requiring a heat treatment in an inert gas. Furthermore, as acontact layer, a cladding layer, an optical guide layer, and the likeare laminated to a total thickness of about 1 μm or more in atwo-dimensional growth mode, the two-dimensional growth progresses basedonly on the ordered lattice array information inherent in the substratesurface. During the growth process, the steps from which thetwo-dimensional growth began will gradually disappear, ultimatelyattaining a very flat uppermost surface. By forming a multiple quantumwell active layer on such a highly flat surface, each layer in themultiple quantum well structure will have a uniform thickness, wherebyemission characteristics are improved. Due to such effects, the p-typecontact resistance can be reduced without particularly performing ap-type properties impartment process, thereby enabling efficient currentinfection. Thus, a long-life light emitting device which experiencesminimum heat generation during use and which has a highly flat surfacecan be provided.

[0046] According to the present invention, a light emitting deviceincluding an active layer which is composed of a plurality of layers ofIn-containing nitride compound semiconductors is produced, using a GaNsubstrate whose crystal orientation is tilted very slightly from the<0001> direction, in such a manner that underlying layers below theactive layer have a total thickness of about 1 μm or more. As a result,a growth mode is realized in which threading dislocations and/ornitrogen vacancies are reduced, and Mg atoms are taken into the mothercrystal by themselves. Thus, it is possible to obtain a p-type crystalwhich is activated to a high density, without performing a heattreatment process after the completion of the device structure which candamage the active layer. As a result, a low resistance p-type contactcan be obtained, thereby improving the longevity of the resultantdevice. By employing the method according to the present invention,threading dislocations are reduced, and hence current paths which do notcontribute to emission are reduced. In addition, the flatness of theactive layer and the underlying layers is improved. Thus, the thicknessof each layer In the InGaN multiple quantum well layers is uniformed,thereby providing improved emission characteristics.

[0047] According to another embodiment of the present invention, inaddition to using the aforementioned slightly-tilted GaN substrate, await period of a predetermined length is observed after a barrier layerand/or a well layer in a multiple quantum well active layer have beenformed. As a result, In atoms which may be taken into the solid phasevia a small number of sparsely present dislocations are prevented frombeing concentrated, thereby obtaining a uniform composition. This effectis enhanced by the reduction of threading dislocations as realized bythe use of a slightly-tilted GaN substrate and forming underlying layersto a total thickness of about 1 μm or more on the slightly-tilted GaNsubstrate. In contrast, a high concentration of threading dislocationsare present in the crystal grown on a conventional substrate, and Inatoms do not diffuse throughout the layers, but rather concentratearound the dislocations because of a small averaged distance betweendislocations. The use of a slightly-tilted GaN substrate as definedabove alone is effective for reducing the concentration of threadingdislocations, but when combined with a wait period as defined above,also eliminated In concentration around dislocations.

[0048] As methods for growing a nitride compound semiconductor, metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE),and halide vapor phase epitaxy (HVPE) are in use. Among others, MOCVDtechniques are generally used from the perspective of crystallinity andproductivity. Hereinafter, examples of semiconductor light emittingdevices according to the present invention will be described.

[0049] (Example 1)

[0050]FIG. 1A is a schematic cross-sectional view illustrating a lightemitting device 1000 according to the present invention. First, a GaNsubstrate 101 whose crystal orientation is tilted away from the <0001>direction by an angle in the range from about 0.05° to about 2° in the<11-20> or <1-100> direction is prepared. In the present specification,a substrate as defined above will be referred to as a “slightly-tiltedsubstrate”.

[0051] Upon the slightly-tilted GaN substrate 101, a GN buffer layer102, an n-GaN layer 103, an n-AlGaN light confining layer 104 forconfining light emitted from an active layer, and a GaN lower opticalguide layer 105 are laminated in this order to a total thickness ofabout 1 μm or more. The GaN buffer layer 102 is optional and can beomitted. The lower optical guide layer 105 may be doped so as to haven-type properties. Further layers that may generally be used for a lightemitting device, e.g., a crack prevention layer and/or a carrier barrierlayer, may be added to the semiconductor light emitting device 1000.Furthermore, the Al component ratio and the electron density in then-AlGaN light confinement layer 104 can be appropriately selected inaccordance with desired device characteristics.

[0052] Upon these layers, an active layer 106 as a light emitting regionis formed, the active layer 106 including a lamination of InGaN barrierlayers 120 (FIG. 1B) and InGaN well layers 121 (FIG. 1B). FIG. 1B showsthe internal structure of the active layer 106 including alternatelayers of the barrier layers 120 and the well layers 121. The Incomponent ratio in the InGaN barrier layers 120 and the InGaN welllayers 121 in the active layer 106, and the number of repetitive unitseach including one InGaN barrier layer and one InGaN well layer 121 canbe arbitrarily selected in accordance with the desired emissionwavelength. An AlGaN layer 107 for preventing evaporation of InGaNand/or diffusion of an acceptor impurity from p-type layers is formedimmediately above the active layer 106. The thickness and the Alcomponent ratio of the AlGaN layer 107 can be arbitrary selected. Insome embodiments, the AlGaN layer 107 may be omitted.

[0053] In contact with the AlGaN layer 107, a GaN optical guide layer108, and a p-AlGaN light confinement layer 109 are laminated in thisorder. The component ratios, thickness, and hole density levels of theselayers can be arbitrarily selected, as is also the case for the GaNlower optical guide layer 105 and the n-AlGaN light confinement layer104. As an uppermost layer that is In contact with the p-AlGaN layer109, a p-GaN contact layer 110 is provided. On the p-GaN layer 110, astripe-shaped p-electrode 112 a is formed with an insulation layer 111interposed therebetween. The insulation layer 111 is provided for thepurpose of current confinement. An n-electrode 112 b is formed on abottom face of the slightly-tilted GaN substrate 101.

[0054] Although the slightly-tilted GaN substrate 101 used An themultilayer structure illustrated in FIG. 1A is of an n-type, it is alsoapplicable to employ a slightly-tilted p-GaN substrate. In such a case,the respective layers in the multilayer structure shown in FIG. 1A havereverse conductivity types (i.e., n-type layers for p-type layers, andvice versa), and the n-electrode 112 b is formed on the uppermostsurface of the light emitting device, whereas the p-electrode 112 aisformed on the bottom face of the slightly-tilted p-GaN substrate 101.

[0055]FIG. 4 is a schematic block diagram illustrating an MOCVDapparatus 2000 used for the production of the light emitting deviceaccording to the present example. In FIG. 4, a GaN substrate 401 is a(0001) plane GaN substrate whose crystal orientation is tilted away fromthe <0001> direction by a slight angle in the range from about 0.05° toabout 2° in the <11-20> or <1-100> direction. The GaN substrate 401 isdisposed on a susceptor 402 made of carbon. In the susceptor 402 isprovided a resistance-heating type heater made also of carbon (notshown). The substrate temperature can be monitored by means of athermocouple and controlled. A water-cooled reaction tube 403 has adouble-tube structure of quartz. Ammonia 406 is used as a group Vmaterial, and trimethylgallium (hereinafter “TMG”) 407 a,trimethylaluminum (hereinafter “TMA”) 407 b, and trimethylindium(hereinafter “TMI”) 407 c are used as group III materials by beingbubbled with nitrogen or hydrogen gas. SiH₄ 409 is used as an n-typedonor material for doping. Bis(cyclopentadienyl) magnesium (hereinafter“CP₂Mg”) 407 d is used as a p-type acceptor material for doping via amaterial inlet 404. The respective materials are introduced into thereaction tube 403, with their amounts being controlled by means of arespective mass flow controller 408, and expelled from an discharge gasoutlet 405.

[0056] Next, an exemplary crystal growth procedure for forming thesemiconductor light emitting device 1000 as a nitride compoundsemiconductor laser/LED will be described with reference to FIG. 1A.

[0057] First, the substrate 101 (corresponding to the GaN substrate 401)is washed and thereafter placed in the crystal growth apparatus(corresponding to the MOCVD apparatus 2000). The substrate 101 issubjected to a heat treatment in an NH₃ atmosphere for about 10 minutesat about 1100° C., and thereafter cooled to a temperature in the rangefrom about 500° C. to about 600° C . Once a stable temperature isachieved, the carrier gas is changed to nitrogen.

[0058] The nitrogen gas is supplied at a total flow rate of about 10l/min, and ammonia is supplied at a flow rate of about 3 l/min. Severalseconds later, TMG is supplied at a flow rate of about 20 μmol/min.Thus, the GaN buffer layer 102 is grown for 1 minute at a relatively lowtemperature so as to have a thickness of about 20 nm. After stopping thesupply of TMG and elevating the temperature to about 1050° C., TMG andan SiH₄ gas are supplied at flow rates of about 50 μmol/min and about 10nmol/min, respectively. Thus, the n-GaN layer 103 is grown so as to havea thickness of about 4 μm.

[0059] Next, TMA is supplied at a flow rate of about 10 μmol/min,thereby growing the n-Al_(0.15)Ga_(0.85)N light confinement layer 104 soan to have a thickness of about 0.5 μm. Note that the light confinementlayer 104 is not required when producing an LED. Next, the supply of TMAis stopped, and the GaN optical guide layer 105 is grown so as to have athickness of about 0.1 μm. The optical guide layer 105 is not requiredwhen producing an LED.

[0060] Thereafter, the supply of SiH₄ and TMG is stopped, and thesubstrate temperature is lowered to a temperature in the range fromabout 850° C. to about 700° C. The substrate temperature at this stageserves as a parameter which determines the emission wavelength of theresultant light emitting device. The lower the temperature, the longerthe emission wavelength. The aforementioned substrate temperature rangeof about 850° C. to about 700° C. is for producing violet to green lightemitting devices; a different temperature range can be used to produce alight emitting device which is outside the violet to green spectrum.

[0061] Once a stable temperature is achieved, TMG and TMI are suppliedrespectively at a flow rate of about 10 μmol/min, thereby forming anIn_(0.05)Ga_(0.95)N barrier layer 120 (FIG. 1B; having a thickness ofabout 5 nm) of the active layer 106. During the growth of the activelayer, SiH₄ may be supplied at a flow rate of about 10 nmol/min. Afterthe growth of the barrier layer 120 is complete, the supply of TMG andTMI is temporarily stopped, and a wait period (from about 1 second toabout 60 minutes) is observed while supplying a carrier gas and an NH₃gas. Thereafter, TMG and TMI are supplied at flow rates of about 10μmol/min and about 50 nmol/min, respectively. Thus, anIn_(0.05)Ga_(0.95)N well layer 121 (FIG. 1B; having a thickness of about3 nm) of the active layer 106. After the growth of the well layer 121,the supply of TMG and TMI is again stopped, and a wait period (fromabout 1 second to about 60 minutes) is observed while supplying acarrier gas and an NH₃ gas.

[0062] The growth of well layers 121 and barrier layers 120 of theactive layer 106 is repeated. After a multiple quantum well structurehaving a desired number layers has been formed, a final barrier layer120 is grown, thereby completing the growth of the active layer 106. Ithas been found that a light emitting device having 2 to 5 well layers121 generally has an optimum emission efficiency.

[0063] After the growth of the active layer 106, TMG (about 10μmol/min), TMA (about 5 μmol/min), and CP₂Mg are supplied, therebygrowing the AlGaN layer 107 (having a thickness of about 30 nm) in orderto prevent sublimation of the InGaN layer. Thereafter, the supply ofTMG, TMA, and CP₂Mg is stopped, and the substrate temperature is againelevated to about 1050° C. After the temperature elevation, TMG (about50 μmol/min) and CP₂Mg are supplied, thereby growing the GaN opticalguide layer 108 so as to have a thickness of about 0.1 μm. The opticalguide layer 108 is not required when producing an LED.

[0064] Next, TMA is supplied at a flow rate of about 10 μmol/min,thereby growing the p-Al_(0.15)Ga_(0.85)N light confinement layer 109 soas to have a thickness of about 0.5 μm. The light confinement layer 109is not required when producing an LED. After completing the growth, thesupply of TMA is stopped, the p-GaN layer 110 is grown so as to have athickness of about 0.5 μm. After completing the growth, the supply ofTMG and CP₂MG is stopped, and the substrate heating is stopped.

[0065]FIG. 5 is a graph illustrating a schedule of growth temperaturesin the vicinity of the active layer and the flow rates of respectivematerials according to the present example of the invention. Illustratedin FIG. 5 are: wait periods 501, barrier layer growth periods 502, welllayer growth periods 503, an n-GaN layer growth period 504, a p-GaNlayer growth period 505, and an AlGaN sublimation prevention layergrowth period 506.

[0066] Once the substrate temperature becomes equal to about roomtemperature, the substrate 101 is taken out from the crystal growthapparatus. A portion of the n-GaN layer 103 is exposed through reactiveion etching, and the insulation layer 111 of a desired configuration,the p-electrode 112 a, and the n-electrode 112 b are formed by a vapordeposition method. The substrate is cleaved to create end faces throughwhich light can be emitted.

[0067] In the case where the semiconductor light emitting device 1000 isformed as an LED, it is unnecessary to form end faces by cleaving thesemiconductor multilayer structure. Rather, light is emitted through thep-electrode 112 a and/or the n-electrode 112 b.

[0068] Although the present example illustrates the use of the GaNbuffer layer 102 as a low temperature buffer layer, this layer may beomitted. Alternatively, Al_(x)Ga_(1−x)N (0≦×≦1) may be used for thislayer, without substantially affecting the production of thesemiconductor light emitting device 1000. The heat treatment in an NH₃atmosphere may also be omitted. It is also possible to apply an elevatedtemperature in an atmosphere of a carrier gas (whose main component isan inert gas) and NH₃, and start the growth of the underlying GaN layer102 concurrently with the introduction of TMG and/or SiH₄.

[0069] In the case of using a GaN substrate 101, it is unnecessary toperform a heat treatment in a hydrogen atmosphere and a growth processfor a buffer layer 102 at a low temperature. It is also possible toapply an elevated temperature in an atmosphere of a carrier gas (whosemain component is an inert gas) and NH₃, and start the growth of theunderlying GaN layer 103 concurrently with the introduction of TMGand/or SiH₄.

[0070] The semiconductor light emitting device 1000 thus producedoperates with a reduced operation voltage and/or current as comparedwith those required for conventional semiconductor light emittingdevices, and yet has a greater emission intensity than that provided byconventional semiconductor light emitting devices. In the case where thesemiconductor light emitting device 1000 is implemented as a laserdevice, unwanted heating is minimized owing to the low contactresistance in the p-type layers, and deterioration occurs only veryslowly, if at all. Thus, a long-life laser device can be realized. Theseadvantages according to the present invention are obtained for thefollowing reasons.

[0071] According to the present invention, a light emitting region isformed on a slightly-tilted GaN substrate 101, so that an organized stepflow growth is realized, substantially reducing threading dislocationsand/or nitrogen vacancies within the crystal. As a result, Mg, as ap-type dopant, can be easily taken into the crystal by itself, i.e.,without being bound to hydrogen atoms. Thus, the acceptor doping layeris imparted with a low-resistance p-type conductivity without requiringany particular post-processing after growth, so that there is no need toperform a heat treatment which would damage the active layer 106.

[0072] Moreover, by forming the underlying layers between the substrate101 and the active layer 106 so as to have a total thickness of about 1μm or more, the steps on the uppermost surface of the substrate 101 aresufficiently flattened, so that it is ensured that the active layer 106attains a uniform structure in which the fluctuation in the thicknessesof the barrier layers 120 and the well layers 121 is minimized.

[0073] Furthermore, reduced threading dislocations and/or nitrogenvacancies and the wait period which is observed during the growthprocess together allow the active layer 106 to have a uniform Incomposition, whereby areas having a high concentration of In atoms,which would be rendered incapable of light emission, can besubstantially eliminated. In particular, immediately after anIn-containing nitride compound semiconductor layer has been grown, thecrystal is not in a sound state because a nitride compound semiconductorcontaining In, when grown at a high temperature, is in a chemicallyunstable state, and dislocations penetrating through the film cause Inatoms to concentrate. Therefore, by forming nitride compoundsemiconductor layers so as to have a total thickness of about 1 μm ormore on a slightly-tilted substrate, the density of threadingdislocations is reduced and the uppermost surface is flattened, and withthe further application of heat in a nitrogen atmosphere, the Inconcentration within the In-containing nitride compound semiconductorlayer is substantially eliminated, whereby a stable phase is obtained.Thus, the crystal attains a sound state. In particular, thecrystallinity of the barrier layers 120 adjoining the wall layers 121,which contribute to the light emission is greatly improved.

[0074] (Example 2)

[0075] In Example 2 of the present invention, a semiconductor lightemitting device 1000 in the form of an LED is produced, using aslightly-tilted substrate 101. In the present example, the relationshipbetween a tilting angle of a slightly-tilted substrate and density ofthreading dislocations present in the semiconductor light emittingdevice 1000, surface roughness, and emission intensity with currentinjection will be discussed.

[0076] By using a GaN substrate 101 having a mirror-polished (0001)plane whose crystal orientation is actually tilted away from the <0001>direction by a slight angle in the range from about 0.02° to about 5° inthe <11-20> or <1-100> direction, a nitride compound semiconductormultilayer structure is grown in the manner shown in Example 1.

[0077] After an n-GaN layer 103 is formed, growth conditions for theactive layer 106 are adjusted so that a constant substrate temperatureis maintained while supplying NH₃. Once a stable substrate temperatureis achieved, TMG, TMI, and SiH₄ are supplied, at flow rates of about 10μmol/min, about 10 μmol/min, and 5 nmol/min, respectively, therebyforming an In_(0.05)Ga_(0.95)N barrier layer 120 (FIG. 1B) within anactive layer 106 so as to have a thickness of about 5 nm. Next, TMG,TMI, and SiH₄ are supplied, at flow rates of about 10 μmol/min, about 50μmol/min, and 5 nmol/min, respectively, thereby forming anIn_(0.2)Ga_(0.)N well layer 121 within the active layer 106 so as tohave a thickness of about 3 nm. After growing the well layer 121, theTMG supply is reduced to about 10 μmol/min, and another barrier layer120 within the active layer 106 is grown. After that barrier layer 120has been grown, another well layer 121 is grown, and so forth; thisprocess is repeated until a final barrier layer 120 is grown.

[0078] Thereafter, an AlGaN layer 107 for preventing sublimation of theInGaN layer is grown so as to have a thickness of about 30 nm, followingthe method described in Example 1. According to the present example, theactive layer 106 includes three well layers 121. After growing the AlGaNlayer 107, a p-type semiconductor multilayer structure is formed, andelectrodes are formed, in the manner described in Example 1. Thus, thesemiconductor light emitting device 1000 as an LED is completed.

[0079]FIG. 6 shows results of a hole density plotting for the Mg dopedlayers of an actual semiconductor light emitting device 1000 produced inaccordance with the above method, the plotting being obtained through ahole measurement before electrodes are provided on the semiconductorlight emitting device 1000. FIG. 7 shows a relationship between thethreading dislocation density in the semiconductor light emitting device1000 according to the present example as evaluated through across-section TEM observation, and the surface roughness as measuredwith a stepmeter. In FIGS. 6 and 7, “” plots represent results obtainedwith a substrate tilt in the <1-100> direction from the <0001>direction; and “603 ” plots represent results obtained with a substratetilt in the <11-20> direction from the <0001> direction. In eithercases, when the tilt angle of the substrate is in the range from about0.020° to about 0.045° or in the range from about 2.1° to about 5°, thetilt in the substrate surface causes crystal malformation, and hence ahigh concentration of threading dislocations and substantial surfaceroughness. As a result, the hole density in those cases was too low tobe measured. Moreover, dot-like regions having a diameter of several nmwere observed in the active layer 106 due to In concentration.

[0080] On the other hand, when the tilt angle of the substrate is in therange from about 0.05° to about 2°, the threading dislocations werereduced, and the hole density was at least 10¹⁷ cm³¹ ³ or more. Thus, itwill be appreciated that a sufficient hole density can be obtained “asgrown” according to the present example of the invention. In addition,the averaged surface roughness of the active layer 106 was reduced toabout 1.8 nm or less, which is sufficiently smaller than the thicknessof each individual well layer In the quantum well structure. Across-section TEM observation revealed that the surface flatness wasalready improved at the time of growing the underlying n-GaN layer 103.Since the reduced threading dislocation density substantially eliminatesthe In concentration in the active layer 106, substantially non-uniformdot-like regions were scarcely observed. Thus, improved flatness of theunderlying layers improved the fluctuation in the thickness of thequantum well structure active layer 106.

[0081]FIG. 8 shows a relationship between the emission intensity and thetilt angle of the substrate tilted in the <11-20> direction or <1-100>from the <0001> direction, in a case where a 20 mA current was flowed inthe semiconductor light emitting device 1000 according to the presentexample of the invention. FIG. 9 shows measurement results of theemission intensity when the growth temperature for the active layer 106was varied between about 700° C., about 750° C., and about 800° C. InFIG. 9, “” plots represent results obtained with a growth temperatureof about 700° C.; “◯” plots represent results obtained with a growthtemperature of about 750° C.; and “Δ” plots represent results obtainedwith a growth temperature of about 800°C. As seen from FIGS. 8 and 9,the emission intensity is enhanced when the tilt angle of the substrateis in the range from about 0.05° to about 2°, although the influence ofthe tilt angle of the substrate on emission intensity has a slightdependence on the growth temperature for the active layer 106. As seenfrom the results shown in FIGS. 7, 8, and 9, there is a clearcorrelation between threading dislocations and emission intensity. Thus,it has been found that the semiconductor light emitting device 1000according to the present invention provides an emission intensity whichis equal to or greater than that provided by a semiconductor lightemitting device produced according to a conventional technique, whilerequiring a smaller driving current. This indicates that the currentpaths not contributing to emission are reduced according to the presentinvention. Although the active layer 106 according to the presentexample is illustrated as including three well layers 121, it has beenfound that similar effects to those provided under the present examplecan be obtained with multiple quantum well structures having two welllayers 121, or any number of well layers between four to ten.

[0082] It has also been found that, in the case where the semiconductorlight emitting device 1000 is produced as a laser device using aslightly-tilted GaN substrate 101 having a tilt angle from about 0.05°to about 2°, the threshold current density at which oscillation beginsis decreased with improved emission intensity, and that emissionintensity for the same level of current is improved relative to thatprovided by a semiconductor light emitting device produced according toa conventional technique.

[0083] (Example 3)

[0084] A semiconductor light emitting device 1000 was produced by amethod similar to Example 2 while varying the thickness of the n-GaNlayer 103, except that the growth temperature for the active layer 106was fixed at about 750° C. FIG. 10 shows results of a surface roughnessplotting for the semiconductor light emitting device 1000 according tothe present example with respect to the total thickness of theunderlying layers between the substrate 101 and the active layer 106.

[0085] In FIG. 10, “” plots represent results obtained with a substratetilt angle of 0.15° in the <1-100> direction from; “◯” plots representresults obtained with a substrate tilt angle of 5° in the <1-100>direction; “♦” plots represent results obtained with a substrate tiltangle of 1.7° in the <11-20> direction; and “⋄” plots represent resultsobtained with a substrate tilt angle of 0.04° in the <11-20> direction.From FIG. 10, it can be seen that, irrespective of the crystalorientation, the flatness of the uppermost surface of the device isimproved with an increased thickness when the tilt angle of thesubstrate is in the range from about 0.15° to about 1.7°. Moreover, inthe case where the semiconductor light emitting device 1000 is producedso that the underlying layers between the substrate 101 and the activelayer 106 have a total thickness of about 1 μm or more, the uppermostsurface has a surface roughness which is smaller than the thickness ofeach individual layer in the quantum well structure. Thus, it has beenindicated that a satisfactory quantum well structure can be obtainedwhen the underlying layers between the substrate 101 and the activelayer 106 have at least a total thickness of about 1 μm in or more.

[0086] Although the present example illustrates various thicknesses ofthe GaN layer 103, similar results were also obtained with underlyinglayers composed of InGaN or AlGaN. The same tendency was also observedin the case where the underlying layers are composed of a plurality ofInAlGaN layers of different compositions; i.e., the flatness of theuppermost surface of the resultant device was improved with a totalunderlying layer thickness of about 1 μm or more, irrespective of thecomposition or the number of layers included.

[0087]FIG. 11 shows a relationship between the emission intensity andthe total thickness of the underlying layers between the GaN substrate101 and the active layer 106 with respect to the semiconductor lightemitting device 1000 produced on a mirror-polished slightly-tilted GaNsubstrate 101 while varying the thickness of the n-GaN layer 103, with a20 mA current being applied via the electrodes. In FIG. 11, “” plotsrepresent results obtained with a substrate tilt angle of 0.150° in the<1-100> direction from; and “◯” plots represent results obtained with asubstrate tilt angle of 0.17° in the <11-20> direction. From FIG. 11, itcan be seen that, in either case, an improved emission intensity isprovided when the underlying layers between the substrate 101 and theactive layer 106 have a total thickness of about 1 μm or more. This ispresumably because the improved flatness of the uppermost surface of thedevice leads to reduced fluctuation in the thickness of the active layer106 and reduced fluctuation in the In component ratio.

[0088] Although the active layer 106 according to the present example isillustrated as including three well layers 121, it has been found thatsimilar effects to those provided under the present example can beobtained with multiple quantum well structures having two well layers121, or any number of well layers 121 between four to ten.

[0089] It has also been found that, in the case where the semiconductorlight emitting device 1000 is produced as a laser device, using aslightly-tilted GaN substrate 101 having a tilt angle from about 0.05°to about 2° in such a manner that the underlying layers between thesubstrate 101 and the active layer 106 have a total thickness of about 1μm or more, the threshold current density at which oscillation begins isdecreased with improved emission intensity, and that emission intensityfor the same level of current is improved relative to that provided by asemiconductor light emitting device produced according to a conventionaltechnique.

[0090] (Example 4)

[0091] In Example 4 of the present invention, a semiconductor lightemitting device 1000 in the form of an LED is produced, using theabove-described growth wait period technique on a slightly-tiltedsubstrate 101. In the present example, the relationship between theemission intensity when a current is injected to the resultant LED andthe waiting time observed after growing each barrier layer 120 in theactive layer 106 will be discussed.

[0092] By using a GaN substrate having a mirror-polished (0001) planewhose crystal orientation is actually tilted away from the <0001>direction by 0.15° in the <1-100> direction, a nitride compoundsemiconductor multilayer structure it grown in the manner shown inExample 1.

[0093] Now, steps for producing the active layer 106 according to thepresent example of the invention will be described with reference to aflow chart shown in FIG. 1C. After an n-GaN layer 103 is formed, growthconditions for the active layer 106 are adjusted so that a constantsubstrate temperature is maintained while supplying NH₃. Once a stablesubstrate temperature is achieved, TMG, TMI, and SiH₄ are supplied, atflow rates of about 10 μmol/min. about 10 μmol/min, and 5 nmol/min,respectively, thereby forming an In_(0.05)Ga_(0.95)N barrier layer 120(FIG. 1B) within an active layer 106 so as to have a thickness of about5 nm (step S130). Next, the supply of TMG, TMI, and SiH₄ is stopped, anda predetermined wait period is observed while supplying a carrier gasand an NH₃ gas (step S131). Thereafter, TMG, TMI, and SiH₄ are againsupplied, at flow rates of about 10 μmol/min, about 50 μmol/min, and 5nmol/min, respectively, thereby forming an In_(0.2)Ga_(0.6)N well layer121 within the active layer 106 so as to have a thickness of about 3 nm(step S132). After growing the well layer 121, the TMG supply is reducedto about 10 μmol/min, and another barrier layer 120 within the activelayer 106 is grown. After that barrier layer 120 has been grown, apredetermined Wait period is observed, and then another well layer 121is grown, and so forth; this process is repeated until a final barrierlayer 120 is grown (step S133).

[0094] Thereafter, an AlGaN layer 107 for preventing sublimation of theInGaN layer is grown so as to have a thickness of about 30 nm, followingthe method described in Example 1. A wait period may or may not beobserved between the growth of the final InGaN barrier layer 120 In theactive layer 106 and the growth of the AlGaN layer 107. However, it hasbeen found that in the case where the active layer 106 includes two orless well layers 121, observing a wait period after the growth of thefinal InGaN barrier layer 120 in the active layer 106 makes for a higheremission intensity responsive to a current injection in thesemiconductor light emitting device 1000. According to the presentexample, the active layer 106 includes three well layers 121.

[0095] After growing the AlGaN layer 107, a p-type semiconductormultilayer structure is formed, and electrodes are formed, in the mannerdescribed in Example 1. Thus, the semiconductor light emitting device1000 as an LED is completed.

[0096]FIG. 12 shows a relationship between the emission intensity andthe waiting time observed after forming each barrier layer 120, In acase where a 20 mA current was flowed in the semiconductor lightemitting device 1000 according to the present example of the invention.FIG. 12 shows measurement results of the emission intensity when thegrowth temperature for the active layer 106 was varied between about700° C. about 750° C., and about 800° C. In FIG. 12, “” plots representresults obtained with a growth temperature of 700° C.; “◯” plotsrepresent results obtained with a growth temperature of 750° C.; and “Δ”plots represent results obtained with a growth temperature of about 800°C. A dotted line in FIG. 12 represents an emission intensity of 400 a.u.(arbitrary units), which is obtained when a zero waiting time isobserved for each growth temperature. The intensity level denoted by the“”, “◯”, or “Δ” symbol represents an average emission intensityassociated with each growth temperature.

[0097] As seen from FIGS. 8, 9, and 12, the emission intensity isfurther enhanced by applying a growth wait period technique in additionto the use of a slightly-tilted substrate 101.

[0098] As seen from FIG. 12, the emission intensity is enhanced when awaiting time of 1 second or more is observed, although the influence ofthe waiting time on emission intensity has a slight dependence on thegrowth temperature for the active layer 106. A relatively long waitingtime provides a significant improvement on the emission intensity in thecase where a low growth temperature is used for forming the active layer106; on the other hand, a relatively short waiting time provides asignificant improvement on the emission intensity in the case where ahigh growth temperature is used for forming the active layer 106.

[0099] Specifically, as seen from FIG. 12, in the case where the growthtemperature for the active layer 106 is about 700° C., a waiting time inthe range from about 1 second to about 60 minutes provides a significantimprovement on the emission intensity, and a waiting time in the rangefrom about 1 second to about 10 minutes provides a particularlysignificant improvement. In the case where the growth temperature forthe active layer 106 in about 750° C., a waiting time in the range fromabout 1 second to about 15 minutes provides a significant improvement onthe emission intensity, and a waiting time in the range from about 1second to about 5 minutes provides a particularly significantimprovement. In the case where the growth temperature for the activelayer 106 is about 800° C., a waiting time in the range from about 1second to about 5 minutes provides a significant improvement on theemission intensity, and a waiting time in the range from about 1 secondto about 2 minutes provides a particularly significant improvement.

[0100] It should also be noted that the above effect was most prominentwhen the time spent for growing each pair of a barrier layer 120 and awell layer 121, including the waiting time observed in between, was inthe range from about 10 seconds to about 120 minutes.

[0101] Although the present example illustrates the use of aslightly-tilted GaN substrate 101 having a mirror-polished (0001) planewhose crystal orientation is actually tilted away from the <0001>direction by 0.15° in the <1-100> direction, It has been confirmed thata tilt in any other direction exhibits similar effects so long as thetilt angle is in the range from about 0.05° to about 2°.

[0102] Although the active layer 106 according to the present example isillustrated as including three well layers 121, it has been found thatsimilar effects to those provided under the present example can beobtained with multiple quantum well structures having two well layers121, or any number of well layers 121 between four to ten.

[0103] It has also been found that in the case where the semiconductorlight emitting device 1000 is produced as a laser device by a methodaccording to the present example, utilizing the above-described growthwait period technique, the threshold current density at whichoscillation begins is decreased with improved emission intensity, andthat emission intensity for the same level of current is improvedrelative to that provided by a semiconductor light emitting deviceproduced according to a conventional technique.

[0104] (Example 5)

[0105] In Example 5 of the present invention, a semiconductor lightemitting device 1000 in the form of an LED is produced, using theabove-described growth wait period technique on a GaN substrate having amirror-polished (0001) plane whose crystal orientation is actuallytilted away from the <0001> direction by 0.15° in the <1-100> direction.According to the present example, a predetermined wait period isobserved after growing each barrier layer 120 within an active layer106, and a predetermined wait period is observed after growing each welllayer 121 within an active layer 106. In the present example, therelationship between the emission intensity when a current is injectedto the resultant LED and the waiting time observed after growing eachwell layer 121 in the active layer 106 will be discussed. Thus,according to the present example, the step of leaving the substrate 101for a predetermined waiting time (step S131) in the flowchart shown inFIG. 1C is performed not only after the formation of each barrier layer120 but also after the formation of each well layer 121. Respectivesteps for forming the active layer 106 according to the present exampleof the invention will be described with reference to a flowchart shownin FIG. 1D. The method for growing each layer in the semiconductor lightemitting device 1000 according to the present example is the same asthat described in Example 3. Hereinafter, growth conditions for formingthe active layer 106 will be described.

[0106] After an n-GaN layer 103 is formed, growth conditions for theactive layer 106 are adjusted so that a constant substrate temperatureis maintained while supplying NH₃. Once a stable substrate temperatureis achieved, TMG, TMI, and SiH₄ are supplied, at flow rates of about 10μmol/min, about 10 μmol/min, and 5 nmol/min, respectively, therebyforming an In_(0.05)Ga_(0.95)N barrier layer 120 within an active layer106 so as to have a thickness of about 5 nm (step S140). Next, thesupply of TMG, TMI, and SiH₄ is stopped, and a predetermined wait periodis observed while supplying a carrier gas and an NH₃ gas (step S141).Thereafter, TMG, TMI, and SiH₄ are again supplied, at flow rates ofabout 10 μmol/min, about 15 μmol/min, and 5 nmol/min, respectively,thereby forming an In_(0.2)Ga_(0.8)N well layer 121 within the activelayer 106 so as to have a thickness of about 5 nm (step S142). Next, thesupply of TMG, TMI, and SiH₄ is stopped, and a predetermined wait periodis observed while supplying a carrier gas and an NH₃ gas (step S143)Thus, after each barrier layer 120 is grown, a wait period is observed,and after each well layer 121 is grown, another wait period is observed,and so forth; this process of alternately forming adjoining layers ofbarrier layers 120 and well layers 121 is repeated until a final barrierlayer 120 is grown (step S144).

[0107] Thereafter, an AlGaN layer 107 for preventing sublimation of theInGaN layer is grown so as to have a thickness of about 30 nm, followingthe method described in Example 1. A wait period may or may not beobserved between the growth of the final InGaN barrier layer 120 in theactive layer 106 and the growth of the AlGaN layer 107. However, it hasbeen found that in the case where the active layer 106 includes two orless well layers 121, observing a wait period after the growth of thefinal InGaN barrier layer 120 in the active layer 106 makes for a higheremission intensity responsive to a current injection in thesemiconductor light emitting device 1000. According to the presentexample, the active layer 106 includes three well layers 121, and thewaiting time observed after growing each barrier layer 120 is about 60seconds.

[0108] After growing the AlGaN layer 107, a p-type semiconductormultilayer structure is formed, and electrodes are farmed, in the mannerdescribed in Example 1. Thus, the semiconductor light emitting device1000 as an LED is completed.

[0109]FIG. 13 shows a relationship between the emission intensity andthe waiting time observed after forming each well layer 121, In a casewhere a 20 mA current was flowed in the semiconductor light emittingdevice 1000 according to the present example of the invention. FIG. 13shows measurement results of the emission intensity when the growthtemperature for the active layer 106 was varied between about 700° C.,about 750° C., and about 800° C. In FIG. 13, “” plots represent resultsobtained with a growth temperature of 700° C.; “◯” plots representresults obtained with a growth temperature of 750° C.; and “Δ” plotsrepresent results obtained with a growth temperature of about 800° C. Adotted line in FIG. 13 represents an emission intensity of 400 a.u.(arbitrary units), which is obtained when a zero waiting time isobserved for each growth temperature. The intensity level denoted by the“”, “◯”, or “Δ” symbol represents an average emission intensityassociated with each growth temperature.

[0110] As seen from FIGS. 8, 9, and 13, the emission intensity isfurther enhanced by applying a growth waiting technique period inaddition to the use of a slightly-tilted substrate 101.

[0111] As seen from FIG. 13, the emission intensity is enhanced when awaiting time of 1 second or more is observed after the formation of eachwell layer 121, although the influence of the waiting time on emissionintensity has a slight dependence on the growth temperature for theactive layer 106. A relatively long waiting time provides a significantimprovement on the emission intensity in the case where a low growthtemperature is used for forming the active layer 106; on the other hand,a relatively short waiting time provides a significant improvement onthe emission intensity in the case where a high growth temperature isused for forming the active layer 106.

[0112] Specifically, as seen from FIG. 13, in the case where the growthtemperature for the active layer 106 is about 700° C., a waiting time inthe range from about 1 second to about 60 minutes provides a significantimprovement on the emission intensity, and a waiting time in the rangefrom about 1 second to about 10 minutes provides a particularlysignificant improvement. In the case where the growth temperature forthe active layer 106 is about 750° C., a waiting time in the range fromabout 1 second to about 15 minutes provides a significant improvement onthe emission intensity, and a waiting time in the range from about 1second to about 5 minutes provides a particularly significantimprovement. In the case where the growth temperature for the activelayer 106 is about 800° C., a waiting time in the range from about 1second to about 5 minutes provides a significant improvement on theemission intensity, and a waiting time in the range from about 1 secondto about 2 minutes provides a particularly significant improvement.

[0113] In the case where a wait period is observed only after the growthof each well layer 121, i.e., without observing a wait period after thegrowth of each barrier layer 120, the emission intensity is somewhatimproved, but to a lesser degree than that illustrated in the graph ofFIG. 12; the improvement in emission intensity is threefold at the most.

[0114] (Example 6)

[0115] In Example 6 of the present invention, a semiconductor lightemitting device 1000 in the form of an LED is produced, using a GaNsubstrate 101 having a mirror-polished (0001) plane whose crystalorientation is actually tilted away from the <0001> direction by 0.15°in the <1-100> direction, following the method described in Example 2,while varying the mixing ratio between a hydrogen gas and a nitrogen gasin a carrier gas which is supplied during a wait period after growing abarrier layer 120 within the active layer 106. In the present example,the relationship between the emission characteristics of thesemiconductor light emitting device 1000 and the mixing ratio between ahydrogen gas and a nitrogen gas in a carrier gas which is suppliedduring a wait period after growing a barrier layer 120 within the activelayer 106 will be discussed.

[0116]FIG. 14 shows a relationship between the emission intensity andemission wavelength of the semiconductor light emitting device 1000 andvarious mixing ratios between a hydrogen gas and a nitrogen gas in acarrier gas which is supplied during a wait period of about 60 secondsafter growing a barrier layer 120, where the total supply rate of thecarrier gas is maintained at a constant level and the growth temperaturefor the active layer 106 is fixed at about 750° C. In FIG. 14, “” plotsrepresent emission intensity (left vertical axis), and “◯” plotsrepresent emission wavelength (right vertical axis).

[0117] As shown in FIG. 14, the emission wavelength and the emissionintensity tend to decrease as the N₂ component 10 in the carrier gasdecreases. The same tendency is also exhibited when the growthtemperature for the active layer 106 is as high as about 800° C. or aslow as about 700° C. In the case where a wait period is observed notonly after growing each barrier layer 120 but also after growing eachwell layer 121, an increased N₂ component in the carrier gas leads toincreased emission intensity and greater emission wavelength.

[0118] (Example 7)

[0119] In Example 7 of the present invention, a semiconductor lightemitting device 1000 in the form of an LED is produced, using a GaNsubstrate 101 having a mirror-polished (0001) plane whose crystalorientation is actually tilted away from the <0001> direction by 0.15°in the <1-100> direction, following the method described in Example 2,while varying the flow rate of an NH₃ gas supplied during a wait periodafter growing a barrier layer 120 within the active layer 106. In thepresent example, the relationship between the emission intensity of thesemiconductor light emitting device 1000 and the flow rate of an NH₃ gassupplied during a wait period after growing a barrier layer 120 withinthe active layer 106 will be discussed.

[0120]FIG. 15 shows a relationship between the emission intensity of thesemiconductor light emitting device 1000 and various flow rates of anNH₃ gas which is supplied during various lengths of wait periods aftergrowing a barrier layer 120, where the growth temperature for the activelayer 106 is fixed at about 750° C. In FIG. 15, “” plots representresults obtained with NH₃ is supplied at a flow rate of about 5 l/min;“◯” plots represent results obtained with NH₃ is supplied at a flow rateof about 3 l/min; and “Δ” plots represent results obtained with NH₃ issupplied at a flow rate of 0 (zero) l/min.

[0121] As shown in FIG. 15, it has been confirmed that the emissionwavelength is enhanced even with a zero flow rate of NH₃; however, thesupply of any non-zero amount of NH₃ has a positive enhancing effect onemission intensity, and also allows for a long wait period to be used,whereby the entire production process can be facilitated. The sametendency is also exhibited when the growth temperature for the activelayer 106 is as high as about 800° C. or as low as about 700° C. Thesame tendency is also exhibited in the case where a wait period isobserved not only after growing each barrier layer 120 but also aftergrowing each well layer 121.

[0122] Examples 1 to 7 chiefly illustrate instances in which the presentinvention is applied to an LED. The inventors have confirmed throughexperimentation that the present invention can be applied to a laserdevice in order to effectively reduce a threshold current value thereof.In fact, semiconductor light emitting devices 1000 which were producedin the form of laser devices, by following the above-described methodsfor realizing high emission intensity in semiconductor light emittingdevices 1000 as LEDs, exhibited laser oscillation with a low thresholdcurrent density. Thus, the present invention is effective for any lightemitting devices generally composed of nitride compound semiconductormaterials.

[0123] As described above, according to the present invention, a crystalis grown on a GaN substrate having a slightly-tilted crystalorientation, whereby a nitride compound semiconductor crystal exhibitinga p-type conductivity which has a hole density of about 10¹⁷ cm⁻³ orabove can be obtained “as grown”, without performing a heat treatment oran electron beam irradiation. As a result, the contact resistance in thenitride compound semiconductor light emitting device according to thepresent invention can be effectively reduced, thereby providing forimproved light emitting device characteristics. By ensuring that theunderlying layers between the surface of the GaN substrate and an activelayer have a total thickness of about 1 μm or more, and observing apredetermined wait period after growing each barrier layer and/or welllayer in a multiple quantum well structure, there is provided ahigh-luminance nitride compound semiconductor light emitting devicewhich can emit light with a relatively low driving current and/oroperation voltage and which is unaffected by heating.

[0124] Various other modifications will be apparent to and can bereadily made by those skilled in the art without departing from thescope and spirit of this invention. Accordingly, it is not intended thatthe scope of the claims appended hereto be limited to the description asset forth herein, but rather that the claims be broadly construed.

What is claimed is:
 1. A nitride compound semiconductor light emittingdevice comprising: a GaN substrate having a crystal orientation which istilted away from a <0001> direction by an angle which is equal to orgreater than about 0.05° and which is equal to or less than about 2°,and a semiconductor multilayer structure formed on the GaN substrate,wherein the semiconductor multilayer structure includes: an acceptordoping layer containing a nitride compound semiconductor; and an activelayer including a light emitting region.
 2. A nitride compoundsemiconductor light emitting device according to claim 1 , wherein theacceptor doping layer comprises Ga_(x)In_(y)Al_(1−(x+y))N (where 0≦x≦1;0≦y≦1; and 0≦x+y≦1).
 3. A nitride compound semiconductor light emittingdevice according to claim 1 , wherein the GaN substrate has a crystalorientation which is tilted away from a <0001> direction in a <11-20> or<1-100> direction.
 4. A nitrite compound semiconductor light emittingdevice according to claim 1 , wherein the acceptor doping layer exhibitsa p-type conductivity as grown.
 5. A nitride compound semiconductorlight emitting device according to claim 1 , wherein the GaN substrateand the active layer are formed so as to be apart from each other by adistance which is equal to or greater than about 1 μm.
 6. A nitridecompound semiconductor light emitting device according to claim 1 ,wherein the active layer has a quantum well structure, and the activelayer has an averaged surface roughness which is equal to or less than athickness of a well layer in the quantum well structure.
 7. A nitridecompound semiconductor light emitting device according to claim 1 ,wherein the active layer includes at least one well layer and at leastone barrier layer.
 8. A method for producing the nitride compoundsemiconductor light emitting device according to claim 7 , wherein themethod comprises: after at least one of the at least one well layer andthe at least one barrier layer has been formed, observing a wait periodduring which no other layers are formed, the wait period having apredetermined length.
 9. A method according to claim 8 , wherein thepredetermined length of the wait period is equal to or greater thanabout 1 second and is equal to or less than about 60 minutes.
 10. Amethod according to claim 8 , further comprising: supplying a carriergas into a chamber, in which the GaN substrate is placed, during thewait period after at least one of the at least one well layer and the atleast one barrier layer has been formed, the carrier gas comprisingnitrogen as a main component.
 11. A method according to claim 8 ,further comprising: supplying a carrier gas and a group V gas into achamber, in which the GaN substrate is placed, during the wait periodafter at least one of the at least one well layer and the at least onebarrier layer has been formed, the carrier gas comprising nitrogen as amain component.